Lumicon 2" Minus Violet Filter - LF3125
Lumicon Minus Violet and Infrared Blocking Filters are used primarily to "bracket" the color-corrected band of refracting telescopes. The Minus Violet and Infrared Blocking Filters can also be used as "visible" filters in RGBV (red-green-blue-visible) astrophotography with reflecting telescopes. The larger Minus Violet filters are for telephoto lenses.
Most digital/CCD cameras have built-in infrared filters, in which case either the Minus Violet or Infrared filter may be used. The choice would depend on how far into the blue your primary lens is color-corrected. The Minus Violet filter blocks wavelengths shorter than about 420nm; the Infrared filter blocks wavelengths shorter than about 400nm. If your camera does not have a built-in infrared filter, the Infrared Blocking Filter is recommended, which blocks all wavelengths longer than 750nm.
The LF3125, 2" Minus Violet Filter, threads into 2" Camera Adapters, such as the Lumicon LC1025, 2" Prime Focus Camera Adapter, or the LA1080, Low Profile 2" Prime Focus Camera Adapter.